Lexicanum is a good source.
'Burning Shore' and 'Temple of the Serpent' are excellent
There is also 'Dragonmage' from the Storm of Magic series, worth reading even though we only feature partially
The army book is pretty good. It details the disagreements of the Slann, the potential heresy of the Cult of Sotek, the different eras etc. Plenty of narrative and characterisation.

It's not really about disproving your impression, opinion is subjective by definition. I just don't agree with your viewpoint. Doesn't mean your wrong necessarily (or I am right), but I'd first read up on Lizardmen before just dismissing them as one-dimensional. The same can be said of any Warhammer race really (they are blatant copypasta from Tolkien mythology after all).
If you want to focus on the 'free will' question, Slann and Skinks are your best bet. They have disagreements between eachother (as in Slann vs Slann, aside from the Prophet we don't know of any other real heresies that have occured, although it's perfectly possible if it happened once). It's also entirely possible to corrupt Skinks, they are sentient beings and are not immune to Chaos (Saurus are so single-minded and loyal it probably wouldn't be worth the effort). Just because GW hasn't been inventive in this field (two books which don't diverge from script much, a handful of references in others) doesn't mean you can't be.

Chaos Dwarves have almost no mentions in current Warhammer lore, but they definitely have character and pathos. Not really a valid argument IMO.

What's wrong with Slann? It would take a good writer (their thought processes are quite alien, so you'd have to think in the abstract to make it work), but Slann are quite independent beings. They debate the course of the future on a daily basis, and magical duels to the death over ideological differences are not unheard of.

Skinks would be substantially easier though I agree, their grasp of language is superior (Slann just tend to telepathically send ideas), and they are much more grounded in their thinking. Still pretty alien mindset though, they are the administrators of a defunct empire in ruins, and have a deep distrust of outsiders. Not to mention Slann have forbidden the presence of the lesser races in Lustria, so striking up a conversation would be problematic.

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With the army book, do you mean the 8th edition Lizardmen book with the lore? or is there actually a book named like that?

I would love to see this band used in Age of SIgmar! We could do something like the Undead have, which allows you to resurrect slain models, seeing as they always say the the warband members never died.

I just reread this link the other day. Really great stuff on Tichi-Huichi and his raiders. Also depicts an almost unknown and abandoned Lizardmen site of Enxilada in the Southlands. Maybe a linke for the fluff-wiki is in order...

I also like how the Arabyans refer to the Lizardmen as Al Saurim. It makes sense that the warmblood-cultures of the Old World or elsewhere would have their own names for the Lizardmen.